This invention relates to a non-aqueous liquid electrolyte secondary cell employing a carbon material as an active material of the negative electrode thereof.
The non-aqueous liquid electrolyte secondary cell, employing a negative electrode of metal lithium and a liquid electrolyte, that is an electrolyte dissolved in a non-aqueous solvent, is low in self-discharge and superior in the operating voltage and storage properties, so that it may be used as a cell for prolonged time with high reliability. For these reasons, cells of this type are extensively employed for timepieces or as a memory backup power source.
The non-aqueous liquid electrolyte cell, so far used as the power source for these devices, is unexceptionally designed as the primary cell. Recently, the development of portable equipment in the field of the electronic equipment, such as video cameras, small-sized audio equipment or micro-computers, is proceeding, and an increasing demand exists for a secondary cell which is lightweight and of a larger capacity and which may be employed economically for a prolonged time as the power source for these portable devices. As the secondary cell capable of meeting the demand, the non-aqueous liquid electrolyte cell is thought to be promising, and research and development is now proceeding towards re-designing the non-aqueous liquid electrolyte cell as the secondary cell.
For example, such non-aqueous liquid electrolyte secondary cells, in which lithium, a lithium alloy or a lithium-occlusive material is used for the negative electrode and MnO.sub.2, TiS.sub.2, MoO.sub.3, MoS.sub.2, V.sub.2 O.sub.5, WO.sub.3 or LiCoO.sub.2 is used for the positive electrode, have been proposed.
Of these, the non-aqueous liquid electrolyte secondary cell, in which a carbon material capable of being doped with and releasing lithium is used for the negative electrode, and a lithium transition metal composite oxides, represented by the formula LiM.sub.x N.sub.1-x O.sub.2, where M, N represent one of Co, Ni, Mn, Cr, Fe or V, above all, lithium cobalt composite oxides, lithium nickel composite oxides or lithium manganese composite oxides, as the positive electrode, are thought to be highly promising because these secondary cells are high in the operating voltage and the energy density and superior in cyclic performance as compared to the non-aqueous liquid electrolyte secondary cell employing metal lithium or lithium alloys for the negative electrode. For example, non-aqueous liquid electrolyte secondary cells employing lithium cobalt composite oxides, lithium nickel composite oxides or lithium manganese composite oxides for the positive electrode, are ideal as secondary cells because their charging/discharging efficiency, represented by the ratio of the charging capacity to the discharging capacity, approaches 100%.
Meanwhile, the non-aqueous liquid electrolyte, comprising an electrolyte dissolved in a non-aqueous solvent, is usually employed as a liquid electrolyte for the non-aqueous liquid electrolyte cell.
If the cell is designed as a primary cell, propylene carbonate, ethylene carbonate, butylene carbonate, dimethoxyethane, tetrahydrofuran or methy tetrahydrofuran is used as the non-aqueous solvents, while LiAsF.sub.6, LiPF.sub.6, LiBF.sub.4, LiCF.sub.3 SO.sub.3, LiCF.sub.3 CO.sub.2 or LiClO.sub.4 is used as the electrolyte.
On the other hand, if the non-aqueous liquid electrolyte cell is designed as the secondary cell used for a portable electronic equipment, such as a video camera, it, is assumed that the charging/discharging is performed at, the charging voltage exceeding 4 V, so that, the non-aqueous liquid electrolyte is required to withstand such higher charging voltage.
However, if the liquid electrolyte employed for the non-aqueous liquid electroylse cell designed as the primary cell is directly used for the non-aqueous liquid secondary electrolyte cell and the charging/discharging cycles are carried out, at the charging volt, age of 4 V, the liquid electrolyte tends to be decomposed, while the active material of the positive electrode tends to be labile and dissolved in the liquid electrolyte, especially in the charged state of the cell, with the consequence that the cell cannot be employed for prolonged time.
For example, if a liquid electrolyte comprising LiClO.sub.4 dissolved in a solvent mixture composed of equal amounts of propylene carbonate and dimethoxyethane at a concentration of 1 mol/liter is employed for the non-aqueous liquid electrolyte secondary cell, and the charging/discharging cycles are repeated under high temperature conditions of not lower than 45.degree. C., the cell capacity is degraded quickly until the cell ultimately becomes unusable.
Consequently, attempts have been made in the non-aqueous liquid electrolyte secondary cell to add a variety of additional solvents to the above-enumerated liquid electrolyte for improving the stability of the cell operation. However, the use of the additional solvents may not be said to be proper in assuring the operational stability because the use of the additional solvents affects the cell performance seriously.